Here is the full list of all 26 Ossuary achievements.
Collect all five treasures
Join all five orders
Learn all five secret words
Enter all five places of power
Uncover all five great lies
Reach all six endings
Expose all five cabbages
Find all 100 obols
Complete the Museum labyrinth
Complete the Order labyrinth
Use the grail
Free all four demons
Learn of the headless hydra
Use a new sin on a new person, 555 times
Receive an answer at the Narthex
Leave the Narthex without an answer
Refuse to think what you must, five times
Tell the District 5 rep that they are disliked five times
Switch parties five times
Kick skull spiders 23 times
Speak to, and use all sins on, all knights in formation
Donate each sin to the museum in turn
Guess wrong on each of the riddler's riddles
Tell the Stockade Guard that the conspiracy is real
Try to touch myself with extra sins after receiving the medal
Learn all sins